How do I calculate the tap drill size for a metric thread?

For ISO metric threads the standard rule of thumb is: tap drill diameter = major (nominal) diameter minus the pitch. For example M6 with a 1.0 mm pitch needs a 5.0 mm drill. This targets roughly 75% thread engagement.

What drill size is needed for an M6 tap?

An M6 coarse thread (1.0 mm pitch) uses a 5.0 mm tap drill. An M6 fine thread (0.75 mm pitch) uses a 5.25 mm tap drill.

What is thread engagement percentage?

It is how much of the theoretical full thread depth is cut. About 75% is the common target: it holds nearly the full strength of the screw while keeping tapping force and tap breakage low. A larger drill lowers engagement and makes tapping easier.

Should I use a larger drill for harder materials?

Yes. In hard or tough materials many machinists drill slightly oversize (toward 50 to 60% engagement) to reduce torque and tap breakage. In soft materials like plastic or aluminium you can stay closer to the chart value.

What is the difference between coarse and fine threads?

Coarse threads have a larger pitch and are the default for general fastening. Fine threads have a smaller pitch, give more holding power in thin material and resist vibration loosening, but need a slightly larger tap drill.

What is the tap drill size for M3, M5, M8 and M10?

For ISO metric coarse threads: M3 is 2.5 mm, M4 is 3.3 mm, M5 is 4.2 mm, M6 is 5.0 mm, M8 is 6.8 mm and M10 is 8.5 mm. The chart in mm above lists every metric and imperial (UNC/UNF) size with its pitch.

What is the difference between a tap drill and a tapping drill?

They are the same thing: tapping drill is the British and Commonwealth term and tap drill the American term for the pilot hole, also called the hole size, that you drill before cutting a thread with a tap. The tapping drill size equals the major diameter minus the pitch.

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Reference

Tap Drill Size Chart

Recommended pilot hole (tap drill) diameters for metric M2–M20 coarse & fine threads and common unified UNC/UNF sizes. Pick a size to get the drill instantly.

Thread size

Tap drill —